Periodical Cicadas

Emergence in Iowa - 1997

By Donald R. Lewis
Extension Entomologist
Department of Entomology
Iowa State University

One of the most interesting insects of North America is the periodical cicada (a.k.a., periodical "locust" and 17-year cicada). Their life span of 17 years includes them among the longest-lived insects in the world. They are also notable for their highly synchronized emergences; all the cicadas in an area emerge at the same time.

Witnessing a massive outbreak takes patience and planning but is worth the wait. Seeing an emergence of periodical cicadas is the "trill of a lifetime."

The periodical cicadas are expected in 46 central, south central and southeast Iowa counties in 1997 (see the accompanying map). They will not be everywhere within those counties, however. The most likely places for these unusual insects is in large, long-standing, upland native woodlands.

The periodical cicadas have spent the past 17 years as nymphs located 18 to 24 inches deep in the soil of wooded and forested areas, feeding on sap from tree roots. The nymphs dig their way out of the soil during late May and June and climb up tree trunks, posts and poles. The outer shell of the nymph splits along the middle of the back, and the winged adult laboriously emerges in about an hour.

Each adult may live for 5 or 6 weeks. During that time the males and females mate and the females lay the eggs that are the start of the next generation. Eggs are laid into the small twigs of trees and shrubs. This causes moderate twig dieback and some disfigurement (flagging) of forest and woodland trees, but no long-term consequences.

The eggs hatch after 6 to 7 weeks and the newly hatched nymphs fall to the ground, burrow until they find a suitable tree root, and begin the feeding and waiting that will last until the year 2014.

Periodical cicadas are well-known for the incredible noise they make when they emerge. The males "sing" with a loud buzzing or drumming sound that goes on all day long. With populations that can reach up to a million and a half cicadas per acre, the sound can be as deafening as it is incessant.

The cicada males sing by vibrating two shell-like drums located along the sides of the abdomen. Strong muscles vibrate the drum membranes several times per second. The resulting high-pitched, rapid clicks are resonated through air sacs and other structures to control sound volume and quality. The upward angles of the wings form a megaphone-like chamber that further controls the sound.

Although this is our largest brood of periodical cicadas, it is not the only brood in Iowa. Other recent periodical cicada emergences occurred in northeastern Iowa (1990) and southeastern Iowa (1985). The current emergence (called Brood III) will occur in Iowa, Missouri and Illinois. Other broods occur in other years in other parts of the eastern United States.

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